To write a fat novel about the Vietnam War nearly 35 years after it ended is an act of literary bravado. To do so as brilliantly as Denis Johnson has in Tree of Smoke is positively a miracle.

This novel makes large demands on the reader: to submit to its length, to its disorienting language and structure, to the elusive and shattering experience of its characters, and finally to its sheer ambition to be definitive an encompassing novel for the Vietnam generation. It is a presumptuous book, in other words, and you may resist for the first several hundred pages. But it will grab you eventually, and gets inside your head like the war it is describing -- mystifying, horrifying, mesmerizing. Johnson, a poet, ex-junkie and adventure journalist, has written a book that by the end wraps around you as tightly as a jungle snake.

Johnson's story revolves around a CIA officer named William "Skip" Sands, who goes to Vietnam in 1967 as part of a team that is running deception operations against North Vietnam. His boss is his uncle, Col. Francis Xavier Sands, a legendary counter-insurgency warrior known to everyone as "the Colonel," and it is the Colonel who hovers over the book like a demon. He is meant to be a mythic character at the heart of darkness -- with a hint of the fictional Kurtz in Conrad's novel and echoes of the real-life Col. Edward Lansdale, the architect of counter-insurgency doctrine in Vietnam.

The black operation that Skip and the Colonel are running is known as "Tree of Smoke." As the novel unfolds, we discover that this may be an attempt to use a Vietnamese double agent to deceive Hanoi into believing that the United States is planning a diabolical attack against the North -- and that the "tree of smoke" may be a mushroom cloud. Johnson includes some interesting tradecraft about running double agents, who as Skip Sands observes, "carry two souls in one body." But the spy-novel machinations are just a subplot. The tree of smoke is the unreal landscape of the war itself.

Fans will recognize Johnson's voice most clearly in Cpl. James Houston and the other soldiers from Echo Recon Platoon, whose nightmarish experiences are woven throughout the book. They are magnificently drawn, their dialogue so sharp and desperate that you are certain this is how soldiers really talked in Vietnam in 1967. Johnson invents a language for them -- a kind of non-stop junkie patter that continues unbroken from the "Floor Show" whorehouse to Echo base camp to bloody battles in the jungle. Like the soldiers in Michael Herr's memoir, Dispatches, Houston becomes a "Lurp," running Long Range Reconnaissance Patrols, which puts him at the most extreme and brutal end of the war. And he loves it, re-ups for another tour, is despondent when he has to go home to Phoenix and become an ordinary loser again. He is addicted to Vietnam, you finally realize. He can't make it anymore in the ordinary world.

This is war as hallucination. It's a story of the decomposition and degradation of the characters and, by implication, Vietnam. A relief worker named Kathy Jones, who is in love with Skip and is in many ways the moral center of the book, warns him that in Vietnam he will ask himself, "When did I die? And why is God's punishment so cruel?" Several hundred pages later, the narrator says, "The life had worn her down," and we see and feel Kathy coming apart. But most of all we see Skip unraveling. He begins the book as an earnest young man who believes all the CIA briefing books; by the end he is a wild outcast running guns in Southeast Asia. "I quit working for the giant-size criminals," he says, "and started working for the medium size. Lousy hours and no fringe benefits, but the ethics are clearer."

The Vietnamese here are timeless, features of a landscape against which the American characters batter themselves senseless. "There's an old saying: The anvil outlasts the hammer," explains one Vietnamese character. "These folks mean business," avers the Colonel. "You whack them down in January, they're back all bright and shiny next May, ready for more of our terrible abuse." They take the beating America inflicts, but they seem impervious to it.

By the end of the book, the major characters are all broken by their versions of Vietnam addiction. "This place is Disneyland on acid," says Sgt. Jimmy Storm, a particularly sadistic operative who is convinced that the Colonel is on the ultimate deception mission when he is actually dead. Before Skip spins out of control, he offers this verdict: "This isn't a war. It's a disease. A plague." That is one of the most powerful themes of the book: Vietnam fed a national craving. We couldn't get out, we couldn't stay in; the war was controlling us rather than the other way around.

Johnson's skill in rendering the dialect of war was earned the hard way -- during the years in which he was, by his own account, a drug addict. He distilled that time in his celebrated collection of short stories, Jesus' Son. He told an interviewer from San Francisco Weekly several years ago that he still liked to go to support meetings and listen to other recovering addicts tell their stories: "I feel very privileged to hear how somebody used to run around stickin' people up and stealing cars, and now they're gettin' their life back together. . . . I just love the stories. The stories of the fallen world, they excite us. That's the interesting stuff." He has used that affinity to capture the rhythms of speech and the mental landscape of the enlisted men who did the fighting.

As a serious war novel, Tree of Smoke is implicitly a story about all wars. And a reader cannot travel this journey without thinking about America's current war in Iraq. Officers and politicians speak of the nobility of this war, as they do of all wars. But when you talk to soldiers in Baghdad or Anbar, you know that it is about surviving, counting down the days, believing in the people on your left and right rather than in the loftier mission statements that emanate from the Green Zone. And those are the lucky soldiers who stay sane. For the vulnerable ones, war takes away these human instincts of survival and replaces them with crazy ones. At the beginning of Tree of Smoke, Cpl. Houston admits that he's scared to death; by the end, he loves kicking other people and being kicked himself.

Something similar must have happened with the mercifully few U.S. soldiers who were involved in America's worst moments in Iraq -- at Abu Ghraib, Haditha and other places we will hear about later. They were damaged people -- addicted to war, feeding on it in a frenzy, being made crazy by it.

President Bush caused a stir not long ago when he said that Iraq was like Vietnam. An incontrovertible statement, surely: We can't get out of Iraq, we can't stay in; the Sunni insurgents who were our biggest enemies are now our best friends; the Shiites for whom we fought the war of liberation are now obstacles to reconciliation. It's a war turned upside down. If we could hear the inner voices of soldiers in Ramadi and Baqubah, behind those wraparound shades they would be thinking about coming home. The decent ones, that is. Those corrupted by war would want to stay on forever, as do Johnson's unforgettable, war-deranged cast of characters. *

David Ignatius is a columnist for The Washington Post and the author of "Body of Lies."